1. Field of the Invention
This disclosure relates to galvanic cells and supercapacitors including electrochemical cells such as electrolysis cells, fuel cells, and batteries, and in particular relates to apparatus and methods for increasing power density of components of galvanic cells and supercapacitors.
2. Description of Related Art
All galvanic cells and supercapacitors that store and/or produce electrical power, comprise three main elements; a positive and negative electrode and an electrolyte that separates them. Everything else exists to accommodate these. Virtually every prior art fuel cell and battery in current literature incorporates the three essential components in a static assembly whereby ions (charged molecules or atoms) move through the electrolyte between electrodes by diffusion. Slow ion diffusion limits power which depends, among other factors, on the rate of ion movement (mass transport) between electrodes.
The electrochemical process for batteries and fuel cells requires polar opposite or oxidation-reduction reactions (hereafter “redox reactions”) to occur on the separated surfaces of solid materials in contact with ionic electrolyte. Useful electrical current is not realized to any sensible extent unless three coordinated processes occur simultaneously. 1) Electricity must flow between the material on or in which it is generated and a current collector (usually metal). Ohm's law applies as much to that current as to any other. 2) Electrical current is thermodynamically generated in accordance with Tafel's law characterized in electrochemistry as resistance, i.e., the negative slope of voltage vs. current. 3) Ions having different electrochemical potential are generated at separated electrodes and in the cases of batteries, fuel cells and electrolyzers must physically meet to neutralize one another. The kinematics of ion mass transport can also be characterized as resistance to electrical current. The three resistances, ohmic, ion kinetics and Tafel are connected in series and dominate the performance of galvanic cells. However, supercapacitors are not much restricted by ion kinetics or the Tafel relationship.
Reactions that involve galvanic or direct current occur mainly on solid surfaces or also to some extent below those surfaces. New and emerging material science technology has produced high concentrations of surface area through nanoscale material development. Consequently, electrochemical activity per weight and volume of current producing material is now very high. However, prior art electrochemical architecture imposes very high resistance due to slow diffusion that restricts ion mass transport kinetics and high ohmic resistance between active material and current collector terminals. Both resistances compel low current density, i/cm2 limitation on prior art cell electrode design because galvanic material must be spread thin over very large current collector area to achieve total energy storage and target power. That is what dominates cell size, weight and cost.
Prior electrochemical cells cannot operate at reasonable efficiency above 1 amp/cm2 or more generally at ½ that. The only way to make a transformational impact on the limitation is to significantly reduce both ion kinetic and electron ohmic resistances simultaneously. When that is achieved it becomes possible to operate the galvanic cell at higher current density with negligible power loss. Batteries use two polar faradaic galvanic materials applied as pastes to metal surfaces containing or immersed in electrolyte and separated by a membrane or electrolyte gap. These materials are only stable in situ under conditions of stationary diffusion but not when exposed to moving electrolyte. Furthermore, pastes must contain conductive additives to support even minimal electrode current density. Stable galvanic coatings of nanoscale substrates (galvanic pellicles) are now possible but these have not been connected to metal current collectors at low ohmic resistance.
A fuel cell is similar except that the particle or surface is a catalyst that does not change its nature to promote a reaction on its surface between fuel or oxygen and electrolyte to produce ions. It may also deal with the complication of a gas phase at its surface. Whether the electrolyte is solid (ionomer) or liquid (e.g., alkali) gaseous fuel must diffuse into electrolyte and become dissolved therein before it is eligible to react on the catalyst surface. That further complicates ion mass transport kinetics since dynamic fuel flow and electrolyte access to catalyst surfaces in a static proton exchange membrane fuel cell (PEMFC) is its own process limitation. But, even if all these issues are solved by convection dynamics, as disclosed in the aforesaid patents, there remains current limiting electrical resistance at the PEM interface with the bipolar plate.
Other electrode combinations of galvanic cell electrodes have been proposed to provide a faradaic battery anode with an air (oxygen) breathing catalytic cathode as a hybrid battery/fuel cell but are not yet realized for high power applications. Water electrolyzers are fundamentally electrolyte convection processes and therefore not amenable to static PEMFC architecture even though the process is an H2/O2 fuel cell in reverse. Supercapacitors are not limited by ion mass transport kinetics because diffusion path length is limited to 1 nm (Helmholtz plane) from electrolyte exposed surfaces. Yet they continue to be current density limited because of high contact impedance at their interface with metal current collectors.
Prior art has embraced carbon nanotubes, CNT as the ultimate in compact surface structure and inherent low resistance electrical conduction material. It is also the material of choice in this invention. CNT is a graphene molecular structure having 10−6 ohm-cm resistivity or about half that of silver metal. The molecular structure of graphene is such that electrons or holes move through the nearly transparent structure as a quantum wave rather differently compared to metal conduction band conductivity. Consequently, when electrical charge must move from CNT to metal it encounters significant ohmic resistance in the transition. Conductivity to the metal is not that of CNT even for CNT vertically aligned with one end attached to a metal surface. Prior art electrode architecture has been devoted to harvesting the CNT and using them to form entangled or nonwoven cohesive mats commonly referred to as ‘buckypaper’. This form, with CNT coated for various purposes is frequently proclaimed to be a major achievement for batteries, supercapacitors and even fuel cells.
For example, U.S. Pat. No. 8,951,697 that issued on Feb. 10, 2015 to Tsinghua University in Beijing China discloses a “pressed carbon tube film” and other carbon fiber modifications as part of a complex CNT “structure” to resolve the high carbon fiber paper resistance. However, significant reductions in CNT contact resistance is not achieved. Additionally, in U.S. Pat. No. 8,021,747 that issued on Sep. 20, 2011 to Samsung, Inc., concerns are raised about the surface resistivity of CNT layers. This Patent discloses a lot of varying methods of altering porosity and pore volume of CNT layers and teaches solutions to many of current resistivity problems, but again fails to significantly decrease contact resistance of CNT layers. It is also noted that U.S. Pat. No. 8,213,157 that issued on Jul. 3, 2012 to the University of Delaware discloses that surface conductivity of a CNT mat is altered by apparent changes resulting in surface modifications from repeated heating and cooling. However, this Patent also fails to achieve a meaningful reduction in contact resistivity of the CNT mat.
Accordingly, there is a need for an improved electrode that minimizes resistivity to current and that also maximizes available surface area on the electrode for galvanic, faradaic or dielectric functions.